Automotive head up displays (HUDs) are typically designed to project a virtual image that appears to be several feet/meters in front of the windshield. For example, BMW states, “The image is projected in such a way that it appears to be about two meters away, above the tip of the bonnet, making it particularly comfortable to read. BMW Head-Up Display halves the time it takes for eyes to shift focus from road to the instruments and back. The system's height can be adjusted for optimal viewing.” This effect is accomplished by collimation, which is the process of making light rays parallel, and may include the adjustment or alignment of optical axes. The trade-off of image collimation is that the eye tends to focus on infinity, i.e., at the same distance as the road ahead. However, inside the vehicle cabin, the optimum image is only viewable within a small spatial area behind the steering wheel, i.e., the “eyebox”. The position of the eyebox is defined by the position of the collimated image reflected off of the windshield.
The optical collimator produces a cylinder of parallel light such that the display can be viewed only while the viewer's eyes are somewhere within that cylinder, a three-dimensional area referred to as the “head motion box” or “eyebox”. Modern HUD eyeboxes are usually about 5 lateral inches by 3 vertical inches by 6 longitudinal inches. These dimensions allow the viewer some freedom of head movement, but movement too far up/down or left/right will cause the display to vanish off the edge of the collimator, and movement too far back will cause it to crop off around the edge. The driver is able to view the entire display as long as one of his eyes is inside the eyebox.
Known HUDs have a fixed eyebox. A driver using an existing HUD would have to keep his eyes within that fixed space in order to see what is being displayed by the HUD.